Imagine a parametric EQ set to a very narrow range, around a specific frequency. Now turn down the gain so you dial out everything at that frequency. That's your "notch". Usually bass players use notch filters to eq out string noise, or the muddiness that occurs in the lower midrange.

For bass the most useful function of a notch filter is to combat the effects of playing live in a room with a low resonant peak allowing you to tune out that one note that is too loud no matter what. I play in one room that has a peak around lo G, even played with the lightest touch is seems to explode.

There is an envelope controlled notch filter on the newer HAZ labs take on the Mutron. I first thought it far to subtle to be useful but eventually tried it with a mid-rich signal and was very impressed? Nothing I hd ever heard B4 which is rare...